Not Applicable
Not Applicable
Not applicable
This invention relates to accessories for Raman spectroscopy and other light-stimulated spectroscopic techniques, such as fluorescence, phosphorescence and light scattering, which prevent operator exposure to intense light sources and also provide standard signals for calibration of the spectroscopic instrumentation.
Raman spectroscopy is a technique for characterizing materials according to the frequency of their molecular vibrations. The basic measurement entails irradiating a sample with a monochromatic light sourcexe2x80x94typically a laserxe2x80x94and analyzing the spectroscopic distribution of the scattered light. Raman spectra are obtained as a series of lines at both higher and lower wavelengths with respect to the exciting source. Typically the spectra are many orders of magnitude weaker in intensity than the exciting source, so the measurements are usually made with relatively intense lasers. These light sources present potential safety hazards to the human eye and skin. Hence, in order for Raman spectroscopic instruments to be used safely, means are required to prevent exposure of personnel to the exciting source beam.
Another requirement of Raman spectroscopic instrumentation is to make measurements with the highest possible accuracy and reproducibility. In order to develop a correlation of the Raman spectra with the molecular bonding and structure of the sample, the wavelengths of the Raman peaks and their relative intensities need to be known with a high degree of certainty. Spectra are measured with a spectrograph that separates the light according to wavelength and directs it onto a detector. Detectors and spectrograph optics, such as diffraction gratings, typically have a response that is wavelength dependent. In addition, the correlation between wavelength and the position of optical elements needs to be determined empirically before the spectrograph can be used for accurate assignment of peak wavelengths. The process of characterizing a spectrograph with respect to the wavelength dependence of measured light intensity and to the correlation between wavelength and position of the optical elements is known as calibration and is carried out using optical standards. Calibration may be effected for example by exposing the input of the spectrograph to standard light sources. To calibrate the instrument with respect to the wavelength dependence of spectral response, a broadband light source with a well-characterized intensity versus wavelength is employed. Calibration for wavelength versus position is usually done by obtaining with the spectrograph spectra of standard materials having one or more sharp spectral lines of precisely known wavelength. For prior art describing calibration of Raman spectrometers, see for example U.S. Pat. Nos. 5,850,623, 5,652,653 and 5,452,084.
Shutters are well-known in prior art as safety features in laser-based instrumentation, and are required by various regulatory agencies. However, as Raman and related optical instruments like fluorimeters, fluorescence microscopes, particle size analyzers and luminometers become more portable, there is a necessity of combining functions. The functions of spectral calibration for wavelength and intensity are typically done by some separate external component such as standard lamps or samples. Instruments need to be calibrated frequently to accommodate minute changes in the positions of optics and detector response that occurs due to environmental factors, component aging, jarring, etc. Thus, there is a general need to include calibration functions within the instrument, and to do so without sacrificing compactness.
A particularly favorable configuration for Raman sampling is the fiber optic Raman probe, see, for example, U.S. Pat. Nos. 5,112,127, 5,911,017, 4,573,761 and 5,978,534. Such probes enable the sample to be at some distance from the spectrograph. In U.S. Pat. No. 5,112,127, for example, laser light is conducted down one optical fiber, focused onto the sample, the scattered light collected with suitable optics, focused into one or more optical fibers, and conducted back to the spectrograph and detector. Fiber optic samplers can present new hazards because of the possibility of the user looking into the distal end. In some fiber optic sampling accessories, the light coming out of the distal end of the sampling apparatus is in a parallel (collimated) form that, on impacting the eye, will become focused onto the retina in a small spot that can readily cause retinal burning. Furthermore, fiber optic samplers are meant to allow users to address samples in small spaces, such as reaction vessels and inside crevices. Hence, it is desirable to build shutters and calibrators into fiber optic sampling accessories without greatly increasing the size and weight.
The object of the invention is to provide an opto-mechanical accessory that can be used with laser-based spectrograph instruments, such as Raman spectrographs, that enables both safe operation and spectral calibration. While the invention can be adapted to most if not all Raman spectrographs, it is particularly adaptable to inclusion into compact fiber optic Raman sampling probes. The basic component comprises a structure with at least two positions, which may be introduced into the optical beam path prior to the sample. The first position contains an opening, an offset assembly or a transmissive optical element such as a neutral density filter that permits the light source to illuminate the sample. The second position contains a material or component that provides a signal that can be used to calibrate the spectrographic instrument. This material/component can be a substance that gives a standard Raman or luminescence spectrum in response to the optical source. The standard spectrum may be recorded whenever a calibration is required. Multiple positions can be used to introduce different standards into the optical path. The open position is used only during spectral measurement of the sample, thus avoiding operator exposure at other times.
The standard could also be an active emissive light source with a known spectral distribution. Possible active light sources include light emitting diodes, semiconductor lasers, incandescent lamps. Thin film organic light emitting diodes are especially compact sources that could be integrated into the assembly of this invention.
The invention is particularly useful as an accessory for Raman spectroscopy. A common component for Raman spectroscopy is the fiber optically coupled sampling probe that can, for example, be immersed in a reaction mixture or process stream to monitor chemical composition. The accessory can be incorporated into such a probe as a sliding bar or rotating wheel for introducing open and calibrate positions into the optical beam. The calibrate position may have a material such as a polyfluorinated hydrocarbon that gives a sharp, characteristic Raman spectrum. It may also have a luminescent material such as commercial green bottle glass that responds to the laser source that provides a broad output of well-known intensity distribution.